Well pump losing flow

[Sprinkler]

Hi, I have a 4" pump (mod SM-3-22) in a 60 meter well.
The water outlet on the pump is 4 cm diameter and there is 4cm pipe from the pump to the well surface. Then there is 2cm (or 2,5 cm ?) pipe from the well to the house.
The pump specifications: L/H 2400 - 600; max lift - 105 meters; RPM - 2850; CV - 1,00 (736 watts).
The issue is that there is a noticeable slowing down of water entering the attic gravity tank. (I do also have a tee joint at the well (with a manual open / close valve) for watering some trees in the garden while simultaneously filling the attic tank.
What do you think may be wrong? Should I take the pump out to inspect for possible semi-blocked water inlet? Is there a way to dismantle the pump for cleaning? I do get some fine silt type sediment in the attic tank.
The pump was installed about 13 years ago, *but* the house has not always been occupied, so I estimate that the pump has accumulated around 4 to 5 years of normal use.
Thanks

 

[Valveman]

If it starts out pumping strong then drops off, the well is likely pumping down, not the pump.

 

[Sponsor]

 

[Sprinkler]

Actually, it seems to be ok. I think it may have been because the trees were getting more water, so the attic tank, less. Probably adjusting the valve to the trees to the optimal position is the solution. Thanks!